The statements in this section merely provide background information related to the present invention and may not constitute prior art.
A typical wash coat includes a catalyst support of gamma, delta, and theta alumina or lanthanum (La) stabilized alumina, Ce—ZrO2 with oxygen storage capacity and Platinum group metals (PGM), such as Pt, Pd, and Rh introduced by nitrate solutions. The PGM can be coated on the cordierite honeycomb substrate combined in the slurry mixture of the catalyst support material and oxygen storage material or the PGM can be coated as separate step following the coating of the wash coat slurry (catalyst support material and oxygen storage material) to make the catalyst. The function of the catalyst is to change most of the pollution of the atmosphere from mobile engine exhaust consisting of carbon monoxide (CO), all kinds of hydrogen carbon compounds (HC), and nitrogen oxygen compounds (NOx) into innocuous carbon dioxide (CO2), water (H2O), and Nitrogen (N2).
As a catalyst support, large fresh specific surface area (SSA) is necessary to supply as many as possible catalytic active sites, meanwhile the thermal stability at high temperature is also very important to maintain catalytic activity of separated sites during operation. High SSA and highly thermal stable La-alumina is a typical catalyst support material.
As an important composition of a three-way catalyst (TWC), zirconia-stabilized ceria and other ceria-based oxides play a major role in oxygen storage and release under lean and rich fuel conditions, thereby, enabling the oxidation of CO and volatile organics and the reduction of NON. High efficient catalytic performance also relates to high specific surface area and thermal stability, as well as high oxygen storage capacity.
Pure ceria is the best oxygen storage material, but its poor thermal stability limits its application at high temperature. Zirconia stabilized ceria, CeO2—ZrO2, has improved the materials' thermal stability and oxygen storage capacity compared with CeO2. The complex oxide with a molar ratio of Ce to Zr of 1:1 can form a solid solution Ce0.5Zr0.5O2 of cubic phase, which improves OSC performance. Nevertheless, the thermal stability of this kind of material is poor after aging at high temperature, for example, after aging at 1000° C. for several hours, the specific surface area of this material will typically be less than 30 m2/g.
Alumina is one example of a thermal stabilizer additive for use with CeO2—ZrO2. Other additives include alkaline earth, rare earth and silicon elements or mixture thereof. The total amount of such stabilizers added is usually controlled to be less than 10 wt. % based on the entire amount of oxides present. However, more recently, up to 90 wt. % alumina has been added into CeO2—ZrO2 for improving thermal stability and OSC. The introduction of alumina into Ce—Zr oxide results in efficient improvement of thermal stability and OSC.
Normally 30-70% Ce—Zr oxide is mixed with La-gamma alumina and platinum group metals (PGM) to make wash coat in a typical TWC application. Ceria-zirconia manufacturing cost is much higher than alumina due to the use of much more organic materials and/or a hydrothermal process. Accordingly, there exists a desire to provide new types of catalyst support materials having high thermal stability and high OSC in order to reduce the usage of Ce—ZrO2 or eliminate the usage of Ce—ZrO2.